There have been wavelength converting devices that obtain ultraviolet rays of converted wavelength from the fundamental continuous visible laser beam by converting the wavelength using non-linear optical materials, such as uniaxial crystal, having optical anisotropy (non-linear optical effect). For example, it has been common practice to convert the wavelength of the light from second harmonic generation (SHG) using non-linear optical materials such as .beta.-barium borate (BBO).
In the wavelength conversion of light from SHG, various non-linear optical materials are used depending on the wavelength and power of the incident light. This is because of the necessity of satisfying the phase matching condition as well as the requirement to use non-linear optical materials with greater effectiveness.
For the sake of simplicity, the following explanation of the wavelength conversion uses the case of SHG. However, the situation is the same in sum frequency generation and difference frequency generation. When the incident light consists of two wavelength components, the wavelength of the emitting light is the sum of both in the former generation, while it is the difference between them in the latter generation.
Once the non-linear optical materials to be used and the wavelength of the incident light are decided for obtaining SHG light with the desired wavelength, the maximum wavelength conversion efficiency factor .gamma. is derived, and the light power P2 for the new wavelength resulting from the conversion is expressed on the basis of incident power P1 as the following equation (1). EQU P2=.gamma..multidot.P1.sup.2
The above equation, however, is applicable only when P2 is sufficiently small compared to P1; as P2 approaches P1, the actual light power has a smaller value than P2. In addition, the above equation is valid only on the condition that the crystal is not damaged by the light power. It is known that the crystal length does not affect the upper limit value of the wavelength conversion efficiency factor .gamma., this is changed by only alteration of the most suitable condition for gathering light (D. Eimerl, et al.: "Optical, mechanical, and thermal properties of barium borate", Journal of Applied Physics, 62 (1987) pp. 1968-1983).
When the incident light power is small, however, the light power at the converted wavelength is weak and often insufficient with the conversion efficiency factor obtained in this way. When non-linear optical materials are used for SHG, the condition for gathering the light is generally such that the convergence angle of the fundamental light nearly equals the acceptance angle (incident angle making wavelength conversion efficiency factor .gamma. maximum) determined by the phase matching condition of the non-linear optical materials.
In the case of critical phasematching (angle phase matching), however, there is considerable anisotropy in the acceptance angle; for example, meridional and sagittal directions differ in acceptance angles. Therefore, the most suitable condition for gathering light requires an elliptic shape of focus that is gathered by a cylindrical lens or the like. This matches the gathering condition to the direction of the smaller acceptance angle; therefore gathering light strongly to the non-linear optical materials with a large incident angle by a so-called spherical lens system. That is, it is impossible to improve the wavelength conversion efficiency by altering the light gathering condition.
On the contrary, in the case of non-critical phasematching (called 90.degree. phasematching condition under certain conditions), the acceptance angle is large and has little anisotropy; therefore, it is possible to gather light strongly to the non-linear optical materials by using a lens system, making it possible to obtain maximum wavelength conversion efficiency by altering the light gathering condition.
In this way, a difference in the maximum conversion efficiency between non-critical and critical phasematching conditions (are proportional to) results from the ratio of acceptance solid angles in the respective conditions. Thus, by means of non-critical phasematching which possibly attain maximum wavelength conversion efficiency, it becomes possible to form a wavelength converting device for SHG with maximum wavelength conversion efficiency factor .gamma..
However, there have been no non-linear optical materials that satisfy the non-critical phasematching condition at optional wavelength desired for SHG; consequently, it must rely on critical phasematching to convert to the light having the wavelength that results in the desired SHG. For this reason, it is difficult to improve the wavelength conversion efficiency by changing the light gathering condition when the incident light has low power.
For the purpose of improving the efficiency with a single element, it might be possible to solve this problem by employing an optical waveguide by which light propagates for a long distance without diffusion; however, it is difficult to apply an optical waveguide to light of high power and it can also be difficult, depending on the material, to construct optical waveguide.
The present invention aims, in view of this situation, to provide a wavelength converting device that can improve wavelength conversion efficiency under the critical phasematching condition.